For example, Steven Pinker looks into this a little in "The Better Angels of Our Nature", long story short there's nothing conclusive, and a sufficiently motivated person can find reasons to doubt it (e.g., the big drop in crime happened some ten years after the ramp up of incarceration rates, Canada experienced a similar decline in crime over that period without the corresponding increase in prison population), however, Pinker himself thinks that when all is said and done it seems likely that incarceration was one of the reasons. One other contributing factor was a large increase in the size of police forces during Clinton's presidency. On top of that, he argues the culture just for some reason shifted to being less violent.

(I want to also mention that he debunks quite convincingly the theory that the decrease in crime had anything to do with legalizing abortion.)